The invention relates to environmental control systems. More specifically the invention relates to an aircraft air conditioning system including an air cycle machine and an electric generator.
An air conditioning system for an aircraft is designed to control airflow into the aircraft's passenger cabin as well as air temperature inside the passenger cabin. Most aircraft air conditioning systems operate on an air cycle refrigeration principle. Compressed air is obtained from an intermediate compressor stage of the aircraft's main engine, cooled with ambient air to near-ambient temperature in an air-to-air heat exchanger and then expanded in an air cycle machine to provide a stream of cooled, conditioned air. The conditioned air is supplied to the passenger cabin. Although somewhat expanded, the conditioned air is still compressed in order to pressurize the passenger cabin.
On occasion, the conditioned air might provide more cooling than necessary. The aircraft might climb to a high altitude, or the ambient air might be very cold. Whenever the conditioned air provides more cooling than necessary, the cooling is reduced by a complex combination of valves and controls.
Additionally, the engine supply pressure might sometimes be greater than required. Whenever this occurs, the pressure is typically reduced by throttling the compressed air. Throttling could be performed by modulating a bleed air pressure regulator valve, a pack flow control valve, or a bypass valve for the air cycle machine. However, throttling is a wasteful process that causes engine fuel consumption to be greater than necessary.
Instead of wasting energy through throttling, the energy can be recovered as disclosed by James Strang et al. in USSN 08/987,737 filed on Dec. 9, 1997 and entitled "Environmental Control System including Air Cycle Machine and Electrical Machine." According to the Strang application, which is assigned to the assignee of the present invention, the air cycle machine is coupled to an electric generator. Cooling capacity and airflow rate are reduced by driving the electric generator and extracting useful electric power. The electric power can be used for operating electric equipment onboard the aircraft.
However, the electric generator provides ac power at variable or "wild" frequencies when the air cycle machine is operated at variable speeds. Such ac power might not be usable by certain electric equipment. Electric equipment such as fuel pumps, environmental controls and recirculation fans might be frequency-sensitive and, therefore, require ac power having a certain range of frequencies or even a fixed frequency. To operate such equipment, the ac power generated by the electric generator is converted to a usable frequency by power conditioning electronics. The power conditioning electronics might include a rectifier for providing dc power to the equipment running on dc power, and an inverter for chopping the dc power to ac power having a fixed frequency.
The power conditioning electronics is expensive. For example, an inverter might cost more than ten thousand dollars. Additionally, the power conditioning electronics is heavy. The inverter might add ninety pounds of weight to the aircraft.
There is a need to reduce the size and weight of the power conditioning electronics.